Poas

Costa Rica

10.2°N, 84.233°W; summit elev. 2708 m

All times are local (unless otherwise noted)

The following is from Boletín de Vulcanología nos. 1 and 2.

Two periods of increased activity have been observed at Poás since February 1978, one in April and May, the other beginning in late September. Ejection of sulfur-rich gray clouds of ash and mud to a height that fluctuated between 5 and 100 m began on 10 April and continued through May. Nine to 50 eruptions occurred per day at intervals of 5-30 minutes. Ejecta fell within the crater or nearby.

Activity was confined to steady gas emission from June until late September. On 22 September explosions resumed, somewhat larger than those of April and May. Gray ash and mud columns were thrown 40-300 m above two vents, one in the center of the crater lake, the other at its S end. Explosions were somewhat less frequent than in April and May; there were up to 30/day at 10- to 45-minute intervals. Much of the ejecta fell within the crater but Volcanology Section personnel visiting Poás on 4 October observed ash deposits averaging 15 cm thick on the E side of the crater. The deposits also contained fine- and medium-grained lapilli and blocks of various sizes. The crater lake, occasionally turquoise green in the past, was entirely gray, and its level had dropped about 3 m, as compared to a 0.5-m drop during the April-May activity. Block ejection and landsliding had produced a vertical wall at the S end of the lake by 4 October.

Geologic Background. The broad, well-vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano, which is one of Costa Rica's most prominent natural landmarks, are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the 2708-m-high complex stratovolcano extends to the lower northern flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, is cold and clear and last erupted about 7500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since the first historical eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.